It is well known that there has been a dramatic increase in recent years in the number of crimes against property committed annually. This wave of crime has tempered the joy of ownership and use with anxiety over the depredations of thieves, robbers, and burglars. Furthermore, many individuals in the apparent safety of their own homes do not feel secure from attacks on their person. For these and other reasons, many people have turned to some form of burglar alarm protection for their homes and property.
A common form of burglar alarm is a system in which a plurality of trip switches are disposed at doors and windows through which forced entry is likely. The switches are wired to a central monitor, and the cost and labor involved in the wiring are a major expense of such a system.
To reduce the amount of wiring required, these systems have been improved by the use of trip switches which plug into electrical wall outlets and send a trip signal through the house wiring to a central monitor. These systems still require wiring from the trip switch to the wall outlet and, not surprisingly, occupy a large number of the available outlets in a home.
Recently much interest and developmental work has focussed on ultrasonic alarms which flood a room (or many rooms) with ultrasonic sound, and use doppler shift to detect motion within the sound field. These systems are generally unsuitable for use in homes, since the rooms of a house are used by the owner and family randomly and casually. Too frequently a homeowner sets off the burglar alarm in his own home through sheer carelessness, merely by opening the door of a protected room.